But this does not mean a new ice age is coming, said astronomer Frank Hill of the US National Solar Observatory. “We have not predicted a ‘little ice age’,” Hill said, speaking from an astronomical meeting in New Mexico. “We have predicted something going on with the sun.”
Hill and other scientists cited a missing jet stream, fading spots and slower activity near the sun’s poles as signs that our nearest star is heading into a rest period.
“This is highly unusual and unexpected,” he said. “But the fact that three completely different views of the sun point in the same direction is a powerful indicator that the sunspot cycle may be going into hibernation.”
That hibernation would not begin now, as the current sunspot cycle, the 24th, has recently passed its minimum. Hill and his colleagues pondered a slowdown in sunspot activity in the 25th cycle, expected sometime around 2019.
They also wondered whether this possible slowdown, or even a long cessation of sunspot activity, indicates an upcoming return of the Maunder Minimum, a 70-year sunspot drought seen from 1645-1715.
They had no evidence as to whether this might be true, and said nothing about whether the Maunder Minimum was related to a long cold period in Europe and other parts of the Northern Hemisphere known as the little ice age.
How strong a connection is there between a little ice age and a Maunder Minimum? “Not as strong a connection as people would like to believe,” Hill said. “The little ice age actually lasted for hundreds of years, of which the Maunder Minimum was only a small segment. My opinion is that there is only an anecdotal connection without a whole lot of scientific background behind it.”
Some commentators have argued that the potentially cooling influence of a lower level of sunspot activity could cancel out the warming caused by human activities that generate climate-warming greenhouse gases. Hill disputed this. “In my opinion, it is a huge leap to an abrupt global cooling, since the connections between solar activity and climate are still very poorly understood,” he said.
Across the sun’s 11-year solar cycle, the total solar energy reaching Earth varies by less than 0.1%, and even across the period since the little ice age chill, solar output climbed no more than about 0.12%, according to the 2007 IPCC report. Subsequent estimates by Judith Lean of the Naval Research Laboratory and others have pegged the solar contribution to 20th-century warming at 10% or less.
Mike Lockwood, professor of space environment physics at the University of Reading, said: “Our research based on the behaviour of the sun over the past 9,000 years shows that there is indeed an 8% chance that we will return to Maunder Minimum conditions over the next 40 years. But there is no evidence at all that this will cause an ice age and, given the observed and predicted rise in greenhouse gases, we find it would do no more than slow global warming a little.”
Prof Joanna Haigh, an atmospheric physicist at Imperial College London, said: “It would certainly be very risky to suggest that we rely on the sun’s activity to compensate for global warming. In a future Grand Minimum the sun might perhaps again cool the planet by up to 1C. Greenhouse gases, on the other hand, are expected to raise global temperatures by between 1.5 and 4.5C by 2100.
“So even if the predictions are correct, the effect of climate change will outstrip the sun’s ability to cool even in the coldest scenario; and in any case, the cooling effect is only ever temporary. When the sun’s activity returns to normal, the greenhouse gases won’t have gone away.”